Dimension reference gauges are used in testing measuring devices such as coordinate measuring machines.
Step gauges corresponding to a plurality of lengths as well as various gauge blocks whose end-to-end dimension is highly accurately calibrated are used for the dimension reference gauge.
The step gauges are comb-shaped components having alternately arranged protrusions and recesses, where a plurality of reference dimensions are defined between end faces of the protrusions. The step gauges are produced by alternately arranging measurement blocks defining the protrusions and spacer blocks defining the recesses and fixing the arranged blocks on a holder. Alternatively, the step gauges are produced by cutting a single component into a form of the comb-shaped component.
A calibration value of the end-to-end dimension of the step gauges is defined as a length at a specific temperature and is often defined as a length at 20 degrees C. (industrial standard temperature).
In testing a coordinate measuring machine, the measured length has to be converted to a length at a temperature during the calibration, which is usually referred to as a length temperature correction. At this time, it is necessary that a coefficient of thermal expansion (CTE) of the step gauge is accurately known.
The CTE used for the temperature correction is written in a calibration certificate or a test certificate for most of the dimension reference gauges including step gauges. Such a CTE is indicated with tolerance.
When a step gauge is used for testing a coordinate measuring machine, the tolerance is considered as a factor of uncertainty in determining uncertainty of the test. Accordingly, it is required that the CTE of the step gauge is highly accurately evaluated in order to reduce the uncertainty in the test.
CTE of an object including a dimension reference gauge is obtained by changing the temperature of the object and measuring a length variation of the object due to the temperature change.
Specifically, a CTE α is given by a formula α=(ΔL/L)·(1/ΔT), where ΔT=T−To (T: a current temperature, To: a reference temperature) represents the temperature variation, and ΔL=L−Lo (L: a length of the object at the current temperature T, Lo: a length of the object at the reference temperature To) represents the length variation (thermal expansion).
In a dimension reference gauge such as a step gauge, the length L of the object is more than 105 times larger than the length variation ΔL. Accordingly, the accuracy of the value of the length L has relatively a small impact on the value of the CTE α.
Accordingly, in order to highly accurately calculate the CTE α, it is necessary to highly accurately measure the temperature variation ΔT and the length variation ΔL.
In order to measure the CTE α, a measuring method using an optical interferometer has been proposed (Patent Literature 1: JP-B-3897655).
In Patent Literature 1, two pairs of optical interferometers opposed on a common measurement axis are used to highly accurately measure an end-to-end dimension of a measurement target (e.g. gauge block). Then, the temperature of the measurement target is changed using a temperature controller to measure the lengths at different temperature, thereby obtaining the thermal expansion due to the changed temperature to calculate the CTE.
However, such a length measurement using the optical interferometer requires a high cost for the optical interferometer. Specifically, not only the optical interferometer per se is expensive, but also reflectivity of air has to be calculated in order to calibrate the wavelength of the measurement light, which entails measurement apparatuses for environment (e.g. temperature, humidity, atmospheric pressure and carbon dioxide concentration). Thus, the cost of the entire system becomes expensive.
Further, since reflection lights from both end faces of the measurement target are used in the length measurement using the optical interferometers, the measurable length is limited to the length of the measurement target. In other words, it is difficult to apply the above method in measurement of dimensions between protrusions at a middle portion of a reference gauge having comb-shaped measurement faces (e.g. a step gauge).
In view of the above problem, a method using a coordinate measuring machine (Patent Literature 2: JP-A-2004-226369) and a method using a pinching unit and a strain gauge (Patent Literature 3: JP-A-2005-83920) have been proposed.
In the method disclosed in Patent Literature 2, a step gauge (measurement target) is disposed in a temperature-controlled chamber. Further, a probe of an external coordinate measuring machine is introduced through an opening of the temperature-controlled chamber and the length of the step gauge is measured using the probe. Then, the temperature setting inside the temperature-controlled chamber is changed to measure the length at different temperature, and the thermal expansion is calculated based on the difference in the measurement lengths before and after changing the temperature.
The length measurement using the coordinate measuring machine does not require an optical interferometer and can be performed as long as a general-purpose coordinate measuring machine is usable in the measurement environment.
Further, the dimensions between protrusions at the middle portion of the step gauge can be measured with the use of the coordinate measuring machine, so that uniformity of thermal expansivity at the middle portion can also be measured.
In the method disclosed in Patent Literature 3, a pinching unit configured to pinch desired one of protrusions of a step gauge and a strain gauge disposed on one of chips of the pinching unit in contact with the step gauge are used, where the temperature is changed while pinching a pair of end faces whose length is to be measured with the pinching unit to directly detect the thermal expansion due to the temperature change using the strain gauge.
It is not necessary to use an optical interferometer in the thermal expansion measurement using the pinching unit and the strain gauge but only simple and inexpensive components (i.e. the pinching unit and the strain gauge) are necessary. Further, the length measurement between protrusions at the middle portion of the step gauge can be performed.
It should be noted that dimension reference gauges (e.g. step gauges) of various dimensions are used in accordance with the size of the coordinate measuring machine to be tested. For instance, a nominal dimension of some of long step gauges exceeds 1.5 meters.
The above-described measurement for the CTE α of the dimension reference gauge is required to be usable for the high-accuracy measurement of the dimension reference gauge with a large length L.
However, the measurable length is fixed by the pinching unit in the above-described method disclosed in Patent Literature 3 and it is not possible to measure a wide variety of lengths (e.g. length of the middle portion). Further, since a pinching unit of a correspondingly large size has to be prepared for a long step gauge, applicability of the method is limited.
Further, since the output of the strain gauge contains noise components, it is difficult to extract only the CTE of the step gauge from the converted length.
In contrast, with the use of the coordinate measuring machine for the length measurement as disclosed in Patent Literature 2, lengths of various parts (e.g. the length between end faces and the length between end faces of protrusions at a middle portion) of the dimension reference gauge of various lengths can be measured, whereby the CTE can be measured based on the results of the measurements at different temperatures.
However, even when a coordinate measuring machine is used for the length measurement as in the method disclosed in Patent Literature 2, measurement accuracy may be deteriorated when the above-described large step gauge with the length of 1.5 m or more is measured.
Specifically, a coordinate measuring machine involves a maximum permissible length measurement error (an index of measurement performance of the coordinate measuring machine). The maximum permissible length measurement error is usually given in a form of a linear expression and becomes larger in proportion to the length to be measured, which means that the larger the length to be measured becomes, the lower the accuracy becomes. A CTE of a long step gauge is difficult to be measured with a high accuracy for the above reason.
Further, some of the dimension reference gauges (e.g. a step gauge) use a section length at a middle portion thereof as a reference length, in addition to the entire length (i.e. an end-to-end distance) thereof. For instance, the section length of the step gauge is defined between end faces of two of the linearly arranged plurality of protrusions at a middle portion.
In order to define a highly accurate reference length as the section length at the middle portion, it is necessary to know a local CTE for the section length.
The CTE in the dimension reference gauge such as a step gauge usually refers to a value obtained by: dividing a thermal deformation over the entire length of the dimension reference gauge by the entire length; and further dividing the obtained deformation per length by a temperature variation before and after the thermal deformation, which is a sort of representative value. However, the CTE sometimes is not even over the entire length in an actual step gauge. For instance, a part of the step gauge in a drawing direction may have high or low CTE. Accordingly, when the section length of the middle portion as described above is defined, it is possible that the CTE of the section is not equal to the CTE (i.e. a representative value) and thus the accuracy of the section length at the middle portion as a dimension reference cannot be ensured.
Accordingly, in order to perform a highly accurate temperature correction in section(s) other than the middle portion in the dimension reference gauge such as a step gauge, it is necessary to measure the CTE for each of the section length(s) at the middle portion to be used as a dimension reference. For the above purpose, the above-described length measurement between the protrusions at the middle portion of the step gauge is of great importance.
Accordingly, it is necessary to house a reference gauge corresponding to a length to be measured in a temperature-controlled chamber together with a measurement target and to set an inside of the temperature-controlled chamber at a predetermined temperature condition. However, when a multiple of intermediate section lengths are defined for the measurement target, corresponding number of reference gauges are required. In addition, complicated processes of exchanging the reference gauge in the temperature-controlled chamber and then setting the temperature-controlled chamber at a plurality of temperatures in order to measure each of the section lengths at the plurality of temperatures are required.
It takes approximately one day in order to stabilize the temperature inside the temperature-controlled chamber at a desired temperature when the temperature-controlled chamber is once opened to exchange the reference gauge. The measurement of CTE accompanying the exchanging of the reference gauge becomes extremely complicated.
Further, in order to save the cost for the equipment and facilitate the preparation process, it is desirable to use existing gauge blocks as the reference gauge. The standard of the gauge block includes “JISB7506” and “ISO3650.” In the specification of the standard, a gauge block having 0.5 mm or more and 1000 mm or less of the entire length is defined. Accordingly, the maximum length of the existing gauge block is 1000 mm. In order to measure a length exceeding the maximum length, it is necessary to prepare a dedicated gauge block with desired accuracy, or to use a plurality of the gauge blocks in combination.
Among the above, preparation of a dedicated gauge block having a length exceeding 1000 mm and a sufficient accuracy entails considerable increase in the measurement cost and thus is not applicable.
On the other hand, the combined use (tightly adhering=wringing) of a plurality of gauge blocks to provide an entire length exceeding 1000 mm is one of originally intended applications of the gauge blocks. However, when a temperature variation occurs as in the measurement of CTE, the tight adhesion of the combined gauge blocks becomes unstable. Specifically, the entire length of the combined gauge blocks may fluctuate depending on the temperature and distortion due to the temperature variation occurs on the mutually adhered faces to release the adhesion. Thus, it has been difficult to use the combined plurality of gauge blocks as a reference gauge for measuring the CTE.
As described above, there are various difficulties in the measurement of the CTE over the entire length of a long dimension reference gauge having a length exceeding a length of a reference gauge, whose solution has been desired.